They have English announcements outside of England so why shouldn't they have Welsh announcements outside of Wales? Welsh people do cross the border you know.
The English announcements “outside of England” are to enable to vast majority of customers on the train to understand the announcement. “Welsh people do cross the border” suggests those people would no longer understand announcements if they were in English. English is also one of two official languages of Wales and the Senedd. According to 2021 census, the Welsh speaking population of Wales aged three or older was 17.8% (538,300 people) and nearly three quarters of the population in Wales said they had no Welsh language skills. Of those Welsh speakers, none don’t also speak English.
It’s only used in Wales because the Welsh government want to keep it going, which is fair enough, but there’s no actual need for Welsh announcements in terms of wayfinding, or certainly in England anyway.
If you board an international train in Europe there will generally be announcements in both languages on both sides of the border.
To make sure everyone can understand where they’re going, like how French passengers on a train that’s just arrived into Italy might not know Italian, but as I said, there is nobody that will arrive into Crewe on a Cardiff Manchester service and require Welsh announcements to know where they are.
Isn't this also an interpretation that it's non-stop to Cardiff rather than stops between Bridgend and Cardiff?
Bridgend was just an example. It does it at Valley before Holyhead, Johnston before Milford Hvn, etc. It’s not a major problem, but it does contribute to yet more noise pollution. “Only” is enough.